Nonprompt direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

The PHENIX collaboration
Phys.Rev.C 109 (2024) 044912, 2024.

Abstract (data abstract)
BNL-RHIC. The measurement of the direct photon spectrum from Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is presented by the PHENIX collaboration using the external photon conversion technique for 0\%--93\% central collisions in a transverse momentum ($p_T$) range of 0.8--10 GeV/$c$. An excess of direct photons, above prompt photon production from hard scattering processes, is observed for $p_T< 6$ GeV/$c$. nonprompt direct photons are measured by subtracting the prompt component, which is estimated as $N_{coll}$-scaled direct photons from p+p collisions at 200 GeV, from the direct photon spectrum. Results are obtained for $0.8<$p_T$<6.0$ GeV/$c$ and suggest that the spectrum has an increasing inverse slope from about 0.2 GeV/$c$ to around 0.4 GeV/$c$ with increasing $p_T$. This indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct photon production, the $p_T$-integrated nonprompt direct photon yields also follow a power law scaling behavior as a function of collision system size. The exponent, $\alpha$, for the nonprompt component is found to be consistent with 1.1 with no apparent $p_T$ dependence.

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